When digital video such as MPEG (Moving Pictures Expert Group) digital video is played back in various trick play modes, access to I frames or P frames is generally required. I frames and P frames contain information that can be used to directly represent an image, whereas images are created with B frames by referencing and modifying preceding and/or subsequent I frames and P frames. As an example, when playback is carried out at 2× or 4× speed, the playback algorithm jumps from I frame to I frame or P frame to P frame to speed up the presentation of the video frames while skipping certain frames.
Unfortunately, when content is encrypted using traditional encryption techniques, trick play becomes difficult or impossible since content has to be decrypted in order to find the I frames and/or P frames. In the case of unencrypted content, these frames are accessed by calculating an offset from the start of frame (SOF).